C# MessageBox1 program

Question

I am a novice to C# program. When I tried to execute the following program following error message is coming. Please anybody explain.

 

“The type or namespace name 'Windows' does not exist in the class or namespace 'System' (are you missing an assembly reference?)”

using System;

using System.Windows.Forms;

public class MessageBox1

{

public static void Main()

{

MessageBox.Show("Hello!");

}

}